The invention relates to a circuit arrangement for powering a two-phase asynchronous motor comprising two windings which each have one end connected to one node and which both have another end connected to a third node, the nodes each being arranged to receive one of three alternating voltages whose sinusoidal fundamental waves at least substantially correspond in frequency.
U.S. Pat. No. 4,829,551 discloses a two-phase drive for a rotating anode of an x-ray tube having a stator comprising two windings: a main winding and an auxiliary winding. The main winding and the auxiliary winding are connected to three nodes, one of which is common to both windings. The three nodes are driven with a main signal, a common signal and an auxiliary signal, respectively, and for this purpose they are connected to switching circuits for applying these signals. According to U.S. Pat. No. 4,829,551 these switching circuits comprise Darlington transistors.
The paper "Phase-Difference Control of 2-Phase Inverter-Fed Induction Motor", CH 2721-9/89/000-0571 (1989) IEEE by Jang, Cha, Kim and Won describes the drive of an induction motor with pulse-width modulated rectangular voltages simulating sinewave voltages.
During operation of a two-phase asynchronous motor the switching circuits for applying the alternating voltages to the nodes are loaded differently in such a manner that the switching circuit connected to the third node must take the highest load, whereas the switching circuits connected to the first two nodes are loaded differently but to a smaller extent depending on the dimensioning of the windings to which they are connected. These different loads lead to different dissipations in the switching circuits, the switching circuit connected to the third node having the highest dissipation as a result of the larger current to be applied thereto. These losses occur not only in the case of a linear power supply with sinusoidal alternating voltages but also occur in the case of power supply with pulse-width modulated rectangular voltages, i.e. in the latter case mainly during the switching operations. Thus, in the case of such a drive the individual switching circuits should be dimensioned for different power dissipations.
On the other hand, standard devices are available which combine three switching circuits for driving the three winding nodes of three-phase asynchronous motors. Preferably, the individual switching circuits of these standard devices are dimensioned identically. If for economical reasons such a standard device is to be used for the construction of a circuit arrangement of the type defined in the opening paragraph the dimensioning of this entire device should be based on the load of the switching circuit connected to the third node. The two other switching circuits are then overdimensioned so that altogether the construction is not economical. Particularly in the case of apparatuses to be manufactured in large quantities at low cost the use of such a switching circuit would be unfavourable.